Systems (personal identification systems) that capture a human face with an imaging device and perform personal identification have been more and more widespread and are beginning to be used for entrance/exit management, login to terminals and the like. With such a personal identification system, there is no risk of theft as compared to a personal identification system that performs personal identification using a password or a portable card, but instead, there is a risk of “impersonation” of impersonating an authorized user in a photograph by illegally obtaining the photograph of the face of the user and holding the photograph over the imaging device. If the impersonation can be automatically detected and appropriately ruled out, the security level of the entire personal identification system can be raised. A number of methods for detecting such impersonation have been proposed (refer to JP-A 2007-304801 (KOKAI); Japanese Paten No. 3822483; T. Mita, T. Kaneko, and O. Hori, Joint Haar-like features for face detection, In Proc. Tenth IEEE International Conference on Computer Vision (ICCV 2005), pp. 1619-1626, Beijing, China, October 2005; Takeshi Mita, Toshimitsu Kaneko, and Osamu Hori, Joint Haar-like features based on feature co-occurrence for face detection, Journal of the Institute of Electronics, Information and Communication Engineers, Vol. J89-D-II, No. 8, pp. 1791-1801, August 2006; M. Yuasa, T. Kozakaya, and O. Yamaguchi, An efficient 3d geometrical consistency criterion for detection of a set of facial feature points, In Proc. IAPR Conf. on Machine Vision Applications (MVA2007), pp. 25-28, Tokyo, Japan, May 2007; Mayumi Yuasa, Tomoyuki Takeguchi, Tatsuo Kozakaya, Osamu Yamaguchi, “Automatic facial feature point detection for face recognition from a single image”, Technical Report of the Institute of Electronics, Information, and Communication Engineers, PRMU2006-222, pp. 5-10, February 2007; and Miki Yamada, Akiko Nakashima, and Kazuhiro Fukui, “Head pose estimation using the factorization and subspace method”, Technical Report of the Institute of Electronics, Information, and Communication Engineers, PRMU2001-194, pp. 1-8, January 2002). According to one of these methods, impersonation using a photograph of a face is detected by using a moving image input to a passive (i.e., non-light-emitting) monocular imaging device to examine the three-dimensional shape of a human face. This method is advantageous in that the device for detecting impersonation does not have to be large-scaled and in being capable of widely applied. For example, in a technique disclosed in JP-A No. 2007-304801 (KOKAI), facial feature points in two images of a captured face with different face orientations are detected and it is determined whether the shape formed by the facial feature points is two-dimensional or three-dimensional.
In the technique of JP-A No. 2007-304801 (KOKAI), however, facial feature points having a large error in a detected position may also be determined to be three-dimensional, that is, an image of a captured face may be determined to be a human face rather than a photograph of the face.